I. Field of the Invention
The present invention relates to communication systems. More particularly, the present invention relates to a method and apparatus for providing closed loop power control in a wireless communication system.
II. Description of the Related Art The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users exist. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known in the art. However, the spread spectrum modulation techniques of CDMA have significant advantages over other modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS," assigned to the assignee of the present invention, and is incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM," also assigned to the assignee of the present invention, and is incorporated by reference herein.
CDMA by its inherent nature of being a wideband signal offers a form of frequency diversity by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a mobile user or mobile station through two or more cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of receivers taking advantage of available path diversity are described in detail in U.S. Pat. No. 5,101,501, entitled "METHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM," and U.S. Pat. No. 5,109,390, entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM," both assigned to the assignee of the present invention and incorporated by reference herein.
If the transmission power of signals transmitted by the base station to a mobile station are too high, it can result in unnecessary interference to other users and reduced system capacity. Alternatively, if the transmission power of signals transmitted by the base station are too low, then the mobile station will receive multiple erroneous transmitted frames. Terrestrial channel fading and other known factors can affect the transmission power of signals transmitted by the base station, so that channel conditions change over time, especially as the mobile station moves. As a result, each base station must rapidly and accurately adjust the transmission power of signals which it transmits to the mobile stations.
In a useful method for controlling the transmission power of signals transmitted by a base station, the mobile station measures the quality of received signals. The mobile station then transmits a signal indicating the measured power level of the received signal, or compares the power level of the received signal to a threshold value and transmits a signal or message to the base station when the power of a received signal deviates from the threshold. In response to the received signal, the base station adjusts its transmission power of the signal to that mobile station; If the power level of the received signal is lower than the threshold, the base station increases the transmission power of its transmitted signals, otherwise, the transmission power is decreased. A method and apparatus for performing power is disclosed in U.S. Pat. No. 5,056,109, entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR TELEPHONE SYSTEM," assigned to the assignee of the present invention and incorporated by reference herein.
A delay necessarily exists under the above closed-loop power control system. The delay includes the time when the base station transmits the frame with inadequate power, when the mobile station receives the degraded or erroneous frame, when the mobile station recognizes the error (e.g., signal falls below the threshold or a frame is erased), when the mobile station transmits an appropriate error message to the base station, and when the base station receives the error message and adjusts its transmission power in response. The digital wireless communication systems, typically, transmit packets of data as discrete frames. As a result, these CDMA communications system cannot rapidly compensate for transmission power fluctuations. Additionally, the mobile station could make multiple requests for increases in the forward link transmission power before recognizing the effects of such requests due to the delay.
Furthermore, the bandwidth of the channel over which the mobile station relays signals to the base station can be limited. For example, nearly all of the bandwidth for this channel is dedicated to transmitting voice traffic to the base station. Little or no bandwidth remains to transmit signals to the base station reflecting the measured power level of the signals transmitted by the base station. Therefore, such signals transmitted by the mobile station could only request that the base station increase or decrease the transmission power by a fixed amount. Consequently, if deep fades affect the transmitted signals, the base station may not be able to react quickly enough to increase its transmission power and calls can be undesirably terminated.